Tetsuya Yamada

High-Molecular-Weight Polyquinoxaline-Based Helically Chiral Phosphine (PQXphos) as Chirality-Switchable, Reusable, and Highly Enantioselective Monodentate Ligand in Catalytic Asymmetric Hydrosilylation of Styrenes

A polyquinoxaline-based helical polymer ligand bearing both helical-sense-determining chiral side chains and coordinating diarylphosphino side chains exhibits solvent-dependent formation of P- or M-helical structures, with which either the S- or R-hydrosilylation product was obtained with high (>93% enantiomeric excess) enantioselectivities.

Third‐order nonlinear optical properties of sol–gel derived α‐Fe2O3, γ‐Fe2O3, and Fe3O4 thin films

The third‐order nonlinear optical properties of sol–gel derived α‐Fe2O3, γ‐Fe2O3, and Fe3O4 thin films have been investigated by the third‐harmonic generation method. Especially, the effects of the valence and coordination number of Fe ions on the third‐order nonlinear optical susceptibility, χ(3), have been examined. The χ(3) values of α‐Fe2O3, γ‐Fe2O3, and Fe3O4 thin films were 5.8×10−11, 2.1×10−11, and 4.0×10−10 esu, respectively, which are the highest values among inorganic oxides reported so far. It was considered that χ(3) of α‐Fe2O3 and γ‐Fe2O3 was enhanced by the pair excitation process involving the simultaneous excitation of magnetically coupled two neighboring Fe3+ ions as well as by virtual transition from O 2p orbitals to Fe 3d orbitals while χ(3) of Fe3O4 was enhanced by both one‐ and three‐photon resonances. The higher second hyperpolarizability, γ(Fex/yO), was obtained when Fe ions are 3+ rather than 2+ and octahedrally rather than tetrahedrally coordinated by oxygens.

Solvent-Dependent Switch of Helical Main-Chain Chirality in Sergeants-and-Soldiers-Type Poly(quinoxaline-2,3-diyl)s: Effect of the Position and Structures of the “Sergeant” Chiral Units on the Screw-Sense Induction

Poly(quinoxaline-2,3-diyl) copolymers bearing various sergeant chiral units with common soldier achiral units have been synthesized to investigate the efficiency of screw-sense induction and its dependence on the nature of the solvents. Optically active 2-alkoxymethyl side chains located at the 6- and 7-positions of the quinoxaline ring induced a single-handed helical conformation more efficiently than 3-methylpentyl or 2-methylbutoxy chiral side chains. Among the 2-alkoxymethyl side chains, those bearing higher 2-alkoxy groups induced a single-handed screw sense more efficiently. For instance, a monomer unit bearing (R)-2-octyloxymethyl groups stabilized the P-helix by 1.01 kJ/mol, whereas the monomer bearing (S)-2-butoxymethyl groups stabilized the M-helix by 0.59 kJ/mol. The effect of the position of the sergeant units in the polymer main chain on the screw-sense induction was also investigated using copolymers in which the positions of the sergeant units were carefully controlled by their synthesis via living polymerization. Chiral units placed sparsely could induce single-handed helical structure efficiently. Chiral units bearing 2-alkoxymethyl, 3-methylpentyl, and 2-methylbutoxy groups showed solvent-dependent helix inversion in CHCl3 and 1,1,2-trichloroethane. No helix inversion was observed in those solvents with chiral units bearing 2-butoxy or (2-methylbutoxy)methyl side chains. The 40-mer of the (R)-2-octyloxymethyl units showed P-helical structures in THF, t-BuOMe, and c-C5H11OMe, toluene, pyridine, Et3N, 1-BuOH, CHCl3, CH2Cl2, 1,4-dichlorobutane, 1,1,-dichloroethane, and 1,1,1-trichloroethane, whereas M-helical structures were induced in 1-BuCN, 1-PrCN, 1,2-dichloroethane, 1,3-dichloropropane, and 2-BuOH.

Catalytic asymmetric synthesis using chirality-switchable helical polymer as a chiral ligand

Single-handed PQXphos, i.e., helical poly(quinoxaline-2,3-diyl)s bearing diarylphosphino pendant groups, served as remarkable chiral ligands in palladium-catalyzed asymmetric hydrosilylation of styrenes and asymmetric biaryl synthesis by Suzuki–Miyaura coupling, affording up to 98 % enantiomeric excess (e.e.) in both reactions. A palladium complex of high-molecular-weight variant (1000mer) of PQXphos could be reused eight times by virtue of the formation of an insoluble polymer complex. PQXphos underwent solvent-dependent inversion of the helical sense, enabling production of either of two enantiomeric products using a single PQXphos.

Majority‐Rules‐Type Helical Poly(quinoxaline‐2,3‐diyl)s as Highly Efficient Chirality‐Amplification Systems for Asymmetric Catalysis

A highly efficient majority-rules effect of poly(quinoxaline-2,3-diyl)s (PQXs) bearing 2-butoxymethyl chiral side chains at the 6- and 7-positions was established and attributed to large ΔG(h) values (0.22-0.41u2005kJu2009mol(-1)), which are defined as the energy difference between P- and M-helical conformations per chiral unit. A PQX copolymer prepared from a monomer derived from (R)-2-octanol (23%u2005ee) and a monomer bearing a PPh2 group adopted a single-handed helical structure (>99%) and could be used as a highly enantioselective chiral ligand in palladium-catalyzed asymmetric reactions (products formed with up to 94%u2005ee), in which the enantioselectivity could be switched by solvent-dependent inversion of the helical PQX backbone.

STM observations of the initial growth processes of metal thin film

The initial stages of Al and Au adsorption onto Si(111) at room temperature have been studied by scanning tunneling microscopy (STM) and reflection high energy electron diffraction (RHEED). During the initial stages of Al deposition three - dimensional epitaxial growth occurred. For films thicker than a few monolayers two - dimensional growth occurred. On the other hand at the initial stage of Au deposition, two - dimensional epitaxial films are formed for the coverage under 2ML. For more than 2 monolayers thickness, three - dimensional growth occurred and the crystallinity changes into the preferential orientation perpendicular to the surface. The effect of ion irradiation has also been studied using partially ionized Au beams. At the acceleration voltage of 1kV small number of the defects on Si (111) substrate were observed.

Al metallization by cluster beam and physical vapor deposition on heavily implanted Si(100) surfaces

Abstract Surface roughening after high-dose ion implantation and annealing was investigated with STM and optical interference methods for high-dose 10 keV B+ and 45 keV As+ implants into Si(100) Al metal films deposited on p+ and n+-Si(100) by cluster beam (CB) and physical vapor deposition (PVD) were characterized with electron diffraction, RBS-channeling and TEM/STM imaging. Epitaxial Al(110) bi-crystalline films were formed by deposition with neutral cluster beams while polycrystalline films were formed by PVD.